I have installed several of them including the one on my 56.
The only issue I can remember is that the pin was too large and had to use the old one. I didn't drill any additional holes in the frame or modify the bracket. I just looked at mine and it mounts parallel to the frame. My set up uses a larger dual diaphragm booster but that shouldn't make any difference. My truck was built in LA also. Perhaps the brackets were not welded correctly if so you may need to modify it to make it fit.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine